A polyurethane is used by virtue of its excellent characteristics as a coating material in the form of a composition suitably prepared by mixing it with other components or reacting it with other compounds.
Examples of such a coating material include
(1) curable coating materials for optical fibers, large cathode ray tubes or cosmetic containers comprising a urethane (meth)acrylate resin which is prepared by the reaction of a urethane with a hydroxylated (meth)acrylate and is curable by irradiation with ultraviolet rays or electron beams,
(2) coating materials of cathodic electrodeposition type which are prepared by the reaction of a urethane with a polyester modified epoxy resin, epoxidized polybutadiene resin or glycidylated acrylic resin and are useful as an undercoat for an automobile body, building skeleton or aluminum sash,
(3) polyurethane coating materials for molded articles of thermoplastics, such as an automobile bumper or a wheel cap or wood works such as furniture, door or wooden parts of office facilities,
(4) one pack baking urethane coating materials for metals which are applicable to outer cases of electrical appliances (such as a refrigerator, audio and VCR), metallic automobile parts, blind and roofing and wall covering materials,
(5) two pack acrylic urethane or polyester urethane coating materials which comprise a polyisocyanate compound prepared by converting an isocyanate into an isocyanurate, biuret or allophanate or an isocyanate adduct prepared by the addition reaction of an isocyanate with a low molecular diol or triol and a hydroxylated acrylic resin or a polyester resin and which are usable in the repair of automobiles or are applicable to automobile parts and wood works, and
(6) urethane powder coatings applicable to outer cases of domestic electrical appliances such as a washing machine or office facilities, and anticorrosive coatings containing a urethane prepolymer useful as an intercoating of a bridge, steel frame or automobile body.
These urethane coating materials are generally prepared by the polyaddition of a polyisocyanate compound with a polyol such as polyether polyol, polyester polyol or low-molecular polyhydric alcohol and, if necessary, by the use of an active hydrogen compound as a chain lengthening agent.
Most of the polyisocyanates to be used in the above polyaddition are directly prepared industrially by the reaction of a polyamine with phosgene, excepting ones having a special structure.
In other words, according to the prior art, at least one of the starting materials to be used in the production of a polyurethane is prepared by the use of toxic phosgene in some preparation step.
In the field of various coating materials described above, the resistances of polyurethane which is one of the main components of the coating materials to weather, corrosion and heat are important factors, so that the development of a polyurethane improved in these characteristics has been expected.
In the case as described above wherein at least one of the starting materials is prepared by the use of toxic phosgene in some preparation step, a polyurethane prepared from the resulting starting materials is inevitably contaminated with chlorine compounds.
The contamination of a polyurethane with chlorine compounds affects the above characteristics, particularly weatherability and corrosion resistance.
A diisocyanate compound directly prepared by the reaction of a polyamine with phosgene generally contains various chlorine compounds such as unreacted phosgene, chloroformate as a by product of the reaction and/or mono- or di-carbamoyl chloride as an intermediate thereof in a concentration of several hundreds of ppm.
Further, these chlorine compounds cannot be removed by any economical process.
Meanwhile, a diisocyanate compound prepared by thermally decomposing a urethane compound prepared by the reaction of a dialkyl carbonate prepared by the use of phosgene with a diamine also contains various chlorine compounds in a concentration of several tens of ppm, though the concentration is lower than that of the diisocyanate prepared by the above phosgene process.
Accordingly, the resistances of a polyurethane to weather and corrosion are fairly improved by the use of this diisocyanate compound, but the improvement is not enough.
Most isocyanate compounds excepting ones having a special structure are prepared directly by the reaction of an amine with phosgene
Among these isocyanate compounds, a diisocyanate compound is reacted with a compound having an active hydrogen such as one having a hydroxyl or amino group to give a polyurethane resin which is very useful in industrial fields as a coating material, thermal insulator, cushioning material, mechanical part or the like and is widely utilized in many necessities for daily life, for example, automobile parts, domestic electrical appliances, business machines, clothing, household furniture and so on.
The following reaction mechanism is described in the Journal of Paint Technology, Vol. 43, No. 562 (1971) as a mechanism of the conversion of isocyanate into urethane in the preparation of a polyurethane resin described above: ##STR1##
Namely, it is presumed that the proton present in the reaction system catalyze the electrophilic attack of the carbonyl group of the isocyanate on a hydroxyl group.
To investigate conventional diisocyanates and processes for the preparation thereof, as described above, most of the conventional diisocyanates are prepared directly by the reaction of an amine with phosgene, so that they contain not only a slight amount of unreacted phosgene but also compounds having a functional group such as a carbamoyl or carboxyl group and hydrogen chloride as impurities.
The content of such impurities in a conventional diisocyanate ranges from 100 to 1000 ppm in terms of chlorine.
Accordingly, it is presumed that the proton eliminated from these impurities catalyzes the conversion of isocyanate into urethane.
However, the content of chlorine compounds in a diisocyanate prepared from a dialkyl carbonate and a diamine is estimated to be as follows. That is, the content of chlorine compounds in a diisocyanate prepared from a dialkyl carbonate prepared through phosgene and a diamine is 5 to 50 ppm in terms of chlorine, while that in a diisocyanate prepared from a dialkyl carbonate prepared without using phosgene and a diamine through a urethane compound is 1 ppm or below. A diisocyanate compound containing such a small amount of chlorine compounds is low in reactivity in its conversion into urethane.
Eager studies have been made in order to enhance the reactivity of such a diisocyanate compound which would be equivalent to or superior to that of a conventional diisocyanate compound directly prepared from phosgene and a diamine and containing about 100 to 1,000 ppm of chlorine compounds.
The mechanism of the conversion of isocyanate into urethane in the preparation of a polyurethane resin is reported in ACS, Symposium, Ser., 270, 111 (1985). ##STR2##
However, a diisocyanate compound prepared from dimethyl carbonate prepared substantially without using phosgene and a diamine through a urethane compound contains impurities different from those contained in a diisocyanate compound prepared by a conventional process, because the former diisocyanate compound is prepared through a reaction route different from that of the latter one.
More specifically, the former diisocyanate contains neither a slight amount of unreacted phosgene nor compounds having a functional group such as carbamoyl or carboxyl group nor hydrogen chloride.
Accordingly, it is impossible to expect the effect of the proton in promoting the formation of a urethane compound shown in chemical formula 1.